Production of single crystal semiconductors

ABSTRACT

Polycrystalline semiconductor material is treated to form a skin of a thermally stable substance and melted with the molten material retained by the film. Upon cooling, the material solidifies as single crystal and the skin is removed.

This application is a division of U.S. Application Ser. No. 290,917filed Aug. 7, 1981.

TECHNICAL FIELD

This invention relates to the production of single crystal semiconductormaterial by forming an enveloping skin around polycrystalline oramorphous semiconductor material and then heating the semiconductormaterial within each enveloping skin to a molten state followed bycooling such that there is formed within the skin a single crystallinebody. The skin is then removed.

BACKGROUND ART

The electrical characteristics of many semiconductor devices aresignificantly improved if the semiconductor material from which they arefabricated is of single crystal structure rather than of polycrystallinestructure. This is especially true in the case of devices whoseperformance improves with increasing minority carrier lifetime.

It is well known that the boundaries between grains in polycrystallinesemiconductor material are often populated by recombination centers formobile charge carriers, and are thus effective in reducing minoritycarrier lifetime. Solar cells comprise one type of semiconductor devicewhose performance improves with minority carrier lifetime. Generally,the photoresponse of semiconductor solar cells is more favorable inlarger grain size material. Material of a single crystal form free ofgrain boundaries is optimum. Semiconductor devices other than solarcells similarly benefit from use of single crystal materials.

To obtain single crystal semiconductor material for fabrication ofdevices, it is common practice in the semiconductor industry to melt orvaporize the material and then cool it in contact with a solid singlecrystal seed of the same or a similar material. As the molten orvaporized material cools, it solidifies epitaxially at the interfacewith the seed such that the single crystal morphology of the seed ispropagated. In this way, the molten or vaporized material is convertedto single crystal form.

In one version of the foregoing process, a seed is slowly withdrawn froma melt and supports a single crystal ingot. In another version, a laseris used to locally melt a layer of polycrystalline material deposited onan underlying single crystal seed substrate. In a third version, thematerial is chemically vapor deposited on a single crystal seedsubstrate usually at an elevated temperature. The main drawback of allthese processes is the requirement for a single crystal seed.

DISCLOSURE OF THE INVENTION

This invention relates to a method for conversion of polycrystalline oramorphous semiconductor material to single crystal form without thebenefit of a single crystal seed. A specific embodiment of this methodthat has been found to operate satisfactorily relates to production ofsmall single crystal spheres of silicon suitable for use in solar cellsor other photovoltaic devices. The invention involves producing singlecrystal semiconductor bodies from irregular bodies of polycrystallinematerial. The material is coated with a skin of dissimilar material; thematerial inside each skin is then melted and thereafter cooled in such away that the molten material solidifies in a single crystal structure.The skin may then be removed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the invention, reference may now be hadto the following description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 schematically illustrates the process of the present invention;

FIG. 2 is a graph illustrating a time temperature profile for theprocess of FIG. 1;

FIG. 3 illustrates a continuous process; and

FIG. 4 illustrates a modification of the invention in which plateletsare produced.

DETAILED DESCRIPTION

Treatment of polycrystalline semiconductor bodies, in accordance withthe present invention, may provide a yield of nearly 100% singlecrystals. The process may be understood by reference to FIG. 1 whichillustrates an apparatus for processing silicon. A hollow inner quartztube 10 is open at end 12 and at the other end is necked down to asmaller diameter portion 14. Tube 10 is nested inside of a larger tube16. Tubes 10 and 16 are maintained in coaxial relation by means ofquartz wool bodies 18 and 20.

A graphite cylinder 22 encircles tube 10 and is located inside tube 16.Cylinder 22 has an inner diameter slightly larger than that of tube 10and an outer diameter slightly smaller than the inner diameter of tube16. The graphite cylinder 22 serves as a heat susceptor when positionedin the annulus between tubes 10 and 16. A radio frequency heating coil24 is positioned coaxial with cylinders 10, 16 and susceptor 22. Itencompasses and heats susceptor 22. A cylinder 26 of quartz woolsurrounds tube 16 between coil turns 28 and 30. Coil 24 is connected toa selectively variable radio frequency power source 32 for controlledheating of susceptor 22.

Section 14 of tube 10 is connected by way of a regulating valve 34 to asource 36 of oxygen so that a predetermined flow of oxygen from source36 is maintained through tube 10. Section 14 of tube 10 is alsoconnected by way of a regulating valve 46 to a source 48 of HCl gas andby way of regulating valve 50 to a source 52 of inert gas. Similarly, asource 38 of nitrogen is connected through a flow regulating valve 40leading to the annulus between tubes 10 and 16 so that the susceptor 22is maintained in a non-oxidizing atmosphere.

A quartz boat 42 is nested in tube 10 at a central location relative tosusceptor 22.

A charge of starting material 44 loaded in boat 42 is preferablyconstituted of semiconductor grade silicon particles doped to thedesired resistivity. The material 44 in boat 42 can be of any crystalstructure such as single crystal, polycrystalline or amorphousstructures. Preferably, the small particles correspond in volume to thatof a sphere of about 0.025 centimeters in diameter. Preferably they willbe near spherical in shape but can be other shapes.

The silicon starting material may be produced by one of many atomizationprocesses such as the process described, for example, in U.S. Pat. No.4,188,177. Suitable particles may also be made by crushingpolycrystalline silicon and screening the same to obtain particles ofdesired size.

Silicon particles of arbitrary crystal structure are placed in boat 42which is positioned in cylinder 10. Energy is then supplied from source32 by way of heating coil 24 while inert gas flows from source 52 andHCl gas flows from source 48 and nitrogen flows from source 38.

In order to clean the particles in boat 42, the temperature of theparticles is elevated to a temperature below the melting point ofsilicon.

One atmosphere of the inert gas Argon mixed with about 5% HCl gas ismaintained for about 10 minutes at a temperature of about 1150° C. Atsuch temperature, the mixture of inert gas and HCl gas removescontaminants from their surfaces.

Valves 46 and 50 are then closed and valve 34 is opened such that oxygenis supplied from source 36. The temperature of the semiconductormaterial in boat 42 is then elevated to a higher temperature thoughstill below the melting point of silicon in order to form an oxide filmon the particle surfaces. The particles preferably are oxidized at oneatmosphere of oxygen for about 25 minutes. The temperature for thisoperation is preferably about 1380° C. At such temperature, while in anoxygen atmosphere, strong oxidizing action takes place and a thinsilicon dioxide layer forms on the outer surface. A silicon dioxide skinor crucible grows on each particle to retain the silicon when later athigher temperatures, it becomes molten.

The particles are then raised to a temperature preferably of about 1430°to 1500° C. but not above 1550° C. at one atmosphere of 0₂ pressure andretained at that temperature for about 2 minutes while melting of thesemiconductor material takes place. This operation is in an atmosphereof oxygen which will act to reseal any rupture that may develop and willotherwise aid in maintaining the integrity of the skin by continuouslyoxidizing the silicon. The skin also prevents adjacent particles fromcoalescing while the selfpackaged silicon is in the molten state.

When molten, each individual particle tends to change its shape tobecome a spherical in character because of the surface tension forceswhich are present. The thin SiO₂ skin is sufficiently plastic at thistemperature to allow the particles to spheroidize.

After the particles have become molten, the heating energy from coil 24is reduced and the molten spheres are cooled.

The cooling rate of the silicon particles through the melting point inthis process is inherently slow which favors single crystal growth.Nearly 100% of the remelted spheres produced by this process are singlecrystal and essentially defect-free.

While spherical particles can be produced with extremely rapid coolingrates, the percentage of polycrystalline spheres produced typicallyincreases as cooling rate decreases. Reasonable yields of single crystalparticles can be obtained by simply withdrawing boat 42 from the furnaceinto an environment at ambient temperature.

Silicon expands upon solidification so that a sphere thus produced mayhave a slight point on the last-to-freeze end of the particle. However,the SiO₂ skin has been found to be sufficiently plastic to allow thesilicon to expand upon solidification without rupturing the skin. Theparticles solidify before significant supercooling occurs and thus ahigh yield of single crystal spheres is produced.

After the boat 42 is removed from the furnace, the skins may be removedchemically by etching in hydrofluoric acid as is well known in the art.

It has been found that several layers of spheres may be placed in boat42. This is possible because the skin grown on the silicon particle willprevent one particle from coalescing with its neighbor and thusparticles may be stacked or piled up. A charge that is several spherediameters deep can be melted without significant sticking of one sphereto another. Actually, a given particle needs to be above 1410° C. foronly a few seconds but for production purposes where particles arestacked, 2 minutes residence time at 1430° C. assures uniformity ofproduct. This is sufficient time to allow all particles to melt andspheroidize.

To facilitate mass production, the particles can be pre-oxidized to formthe skin thereon in a preconditioning furnace or in a cooler portion ofa single furnace. However, in FIG. 1, only a single stage furnace hasbeen shown. It is to be noted that HCl gas/inert gas pretreatment maynot be required if the particles are extremely clean at the start.

When processing large quantities of silicon spheres at a given time, itmay be desired to stir the spheres during the oxidation stage so thatall surfaces of the particles are uniformly oxidized. They must not bestirred while molten.

The process of melting silicon in an SiO₂ skin results in theincorporation of large quantities of oxygen into the silicon. This candegrade the electrical properties. To eliminate this problem, the oxygencontent of the particles can be reduced by heating the spheres in anoxygen ambient at 1250° C. for 3-5 hours. This can be done simply bylowering the temperature of the melting furnace or, to facilitate massproduction, by annealing in a separate furnace.

If oxygen removal is carried out in a separate furnace, the sphereswhich are cooled slowly to a point below the melting point of siliconshould be rapidly cooled and later should be reheated rapidly to avoidformation of nuclei upon which the dissolved oxygen will precipitateduring subsequent heat treatment. If oxygen is not removed, itprecipitates on further thermal processing to form oxygen precipitateinduced stacking faults throughout each particle and the latter havebeen found to be the source of degradation.

If the oxygen is removed by annealing, the minority carrier lifetimewill remain high. In such case, the photovoltaic response of the siliconsphere will not be affected by oxygen.

Thus, in this process, silicon particles in the shape of spheres or ofother shapes are simply loaded onto a flat boat and moved through afurnace controlled in temperature. The furnace ambient is purged withthe reactive gas that grows a skin on each particle. The hightemperature reaction in the furnace causes the skin to form and then allthe silicon particles are made to be molten. With oxygen gas used as thereactive gas before melting, a silicon dioxide film is formed on eachparticle from the furnace oxygen ambient. During melting, the outsideskin remains intact and retains the molten silicon. The thin silicondioxide skin is plastic at this temperature, allowing the droplet tospheroidize. The surface also loses texture and becomes shiny smooth.

It has been found desirable to coat the quartz boat 42 with Si₃ N₄ tominimize adhesion of the spheres to the boat.

It will be understood that other furnace construction materials such asAl₂ O₃, SiC, Si₃ N₄, etc. may be employed in place of quartz. Further,the furnace may be resistively heated rather than RF heated as abovedescribed.

While use of oxygen gas has been described as the oxidizing ambient inthe furnace, N₂ O or other gases may also be suitable. Si₃ N₄ or SiCskins may also be formed with the appropriate nitrogen or carbon bearinggases employed in tube 10.

FIG. 2

FIG. 2 illustrates time-temperature relations employed in treatingsilicon.

Referring to FIG. 2, silicon particles are first heated to a temperureof about 1150° C. for about 10 minutes. The temperature in the furnaceis then elevated to about 1380° C. and maintained at the temperature of1380° C. for about 3 minutes. The temperature then is raised preferablyto about 1430° C. which is 20° above the melting point of silicon and ismaintained for about 2 minutes. The temperature is then reduced to 1380°C. and held for about 10 minutes. The temperature then is permitted toreturn to ambient as indicated by the dotted line 60.

Alternatively, the temperature in the furnace may be reduced to about1250° C. as indicated by the dotted line 61 and the particles held atthat temperature for a period of several hours. Practical and productiveutilization of the furnaces suggests that the charge rendered molten andthen permitted to cool should be returned to ambient temperature.Thereafter the charge would be transferred to a different carrier andplaced in a different furnace which would anneal the particles at 1250°C. for several hours, as indicated by the dotted line 62. Thus, FIG. 2illustrates two alternatives, (1) where a single furnace is used and (2)where one furnace is used to form the skin and melt the silicon withinthe skin, followed by slow cooling to a temperature below the meltingpoint and then rapid cooling down to ambient temperature to be followedby annealing in a second furnace.

FIG. 3

In FIG. 3, a continuous process is illustrated. The furnance of FIG. 3comprises an inner compartment having a bottom wall 110 and a top wall112. The inner compartment is surrounded by an outer compartment havinga bottom wall 114 and a top wall 116. The compartments may berectangular with substantial widths and need not be cylindrical as wasthe case of FIG. 1. Susceptor structures 117 and 119 are located in thespaces between the inner and outer compartments. Oxygen is fed into theinner compartment by way of tubes 118 and 120. Nitrogen is fed into thespacing between the inner and outer compartments by way of tubes 122 and124. A quartz wool body 126 coats or surrounds the outer compartment. Afirst RF heating coil 128 is powered from an RF supply source 130. Asecond RF heating coil 132 is powered from an RF supply source 134.

A plurality of boats 140-154 travel on a track inside the innercompartment. The boats are propelled by a feed unit comprising a drivenbelt 158 having cleats 160 which engage the trailing end of each boat asit is placed in position to enter the furnace. Boat 156, for example, isshown in position to be placed onto belt 158 as boat 154 enters thefurnace through an end gate. The boats may travel at a uniform ratethrough the furnace. The furnace is adjusted by control of the heatingcoils 128 and 132 so that the first section within susceptor 117 andcoil 128 will be maintained at about 1380° C. for 25 minutes and boatswithin susceptor 119 and coil 132 will be maintained at a temperature of1430° C. for about 2 minutes, following which they emerge from thefurnace, whereupon the particles are cooled.

While the foregoing example has dealt with the treatment of silicon toproduce single crystal silicon spheres, it is to be understood thatother semiconductors may be similarly treated.

The skin should be such that it is not rapidly dissolved by thesemiconductor and has plasticity at the melting point.

In forming single crystal germanium, the gas in the furnace during theperiod of growth of the skin would be silane (SiN₄) plus anhydrousammonia (NH₃) plus Argon. The growth of the skin sould involvesubjecting the germanium particles to the above gases at 800° C. forabout 5 minutes then to a temperature of 950° C. for about 2 minutesthen at a temperature of about 800° C. for a few minutes and then toambient temperature.

Further, the semiconductor gallium arsenide may be produced in singlecrystal form. In such case, silicon nitride, silicon dioxide or siliconcarbide may be applied by chemical vapor deposition to form the skin.

The foregoing description has dealt with the production of semiconductorspheres of single crystalline form. It is to be understood that singlecrystals of shape other than spherical can be produced within the scopeof the present invention.

FIG. 4

FIG. 4 illustrates single crystal semiconductor platelets formed on asubstrate. A substrate 200 of silicon nitride or quartz or like materialis first coated with a continuous coat of a semiconductor material suchas silicon about 5 to 10 mils thick. The silicon is then etched inconventional manner to form islands of separate and distinct plates--forexample, plates one-quarter of an inch square on the upper face of thesubstrate 200. Platelets 201-205 are shown on the upper surface of thesubstrate 200.

Thereafter, in accordance with the present invention, atime-temperature-reactant regime such as illustrated in FIG. 2 isfollowed whereby a coating such as coating 206 is formed on each of theplatelets 201-205. After forming the coating 206, the temperature of theplatelets is elevated so that they become molten but are retained intactby the action of the skins 206. After melting, the temperature isreduced as shown in FIG. 2 so that the particles are solidified and thenare reduced to ambient temperature. The substrate may then be diced sothat single crystal platelets 201-205 are available for such use as maybe desired of such a product. Integrated circuits may be formed thereonor other desired operations are carried out where use of single crystalplatelets is advantageous.

Having described the invention in connection with certain specificembodiments thereof, it is to be understood that further modificationsmay now suggest themselves to those skilled in the art and it isintended to cover such modifications as fall within the scope of theappended claims.

We claim:
 1. A particle of single crystal semiconductor materialencompassed within a skin comprising a thermally stable compound formedby the reaction of said material with a reactive gas at an elevatedtemperature.
 2. The particle according to claim 1 wherein said particleis in platelet form on a substrate with said free surfaces of saidmaterial covered by said skin.